1. Field of the Invention
This invention relates to analog storage. In particular, the invention relates to analog non-volatile flash memory.
2. Description of Related Art
Analog storage has been used in a number of recording and playback applications. For example, U.S. Pat. No. 5,220,531 issued to Blyth and Simko describes an analog storage scheme using electrically erasable programmable read only memory (EEPROM). Such an analog storage typically has memory cells using floating gate technology. A memory cell of a floating gate device has a source, a drain, a gate, and a floating gate. The threshold between the gate and the source is determined or controlled by the charge on the floating gate. These memory cells are erased using Fowler-Nordheim tunneling by applying a high voltage, e.g., 21 Volts, on the gate, zero volt on the source, and zero volt on the drain. The memory cells are programmed using the same Fowler-Nordheim tunneling mechanism by applying a high voltage (e.g., 9 to 19 volts) on the drain, zero volts on the gate, and 6 volts on the source.
Existing techniques for programming analog storage cells have a number of drawbacks. First, the programming is slow because the iterative sequence uses constant program pulses having constant programming voltage. To achieve high accuracy, the programming voltage step size has to be sufficiently small. Iterative programming the memory cells using small programming voltage step size requires a large number of steps, resulting in slow programming time. Second, the programming is used for EEPROM memory cells, and therefore is not applicable for flash memory cells which have different programming requirements.
Therefore, there is a need to have an efficient and accurate technique to program flash memory cells.